· Update May 4th (comments updated June 27th because I'm easily distracted): small revisions, 1 new unique comic, now available as an Imgur album
· Update April 3rd 12:45am PST: 81% longer; more of everything
· Early April 1st through April 1st 5:15pm: constant updates
· Comics and location data scoured from all 2025+ comments, across 5 reddit threads and 1 xkcd forum thread. Haven't read all of the xkcd forum theories.

Okay, I think I figured it out. The alt-text reads:

Umwelt is the idea that because their senses pick up on different things, different animals in the same ecosystem actually live in very different worlds. Everything about you shapes the world you inhabit --from your ideology to your glasses prescription to your browser window size.

As geary and same3chords (via googledocs) mentioned, it's using the referrer (your "ideology"), and/or geographic location, browser, ISP, and OS to determine which comic you see. Many comics also change depending on the magnification of your browser (your "glasses prescription") or your browser's window size (your "browser window size"... okay that one was straightforward).

The different versions:

Unknown Correlations. Theories:

Legend:

Lo Not solely location-based

Br Not solely browser-based

Very Common. Unknown - Snake - Variable by width (composite of all versions)
· Alt text: [original]
· Correlation: Lo, Br Seems to come from everywhere
· Like several other comics, this one changes depending on the resolution width of your browser (based on combined window size with magnification). It is made up of four panels, where the first one is always the same, the second has five variations, third: four variations, fourth: seven variations (for a total of 17 panels, creating up to 140 full-comic variations) to create the desired width. (Note: will collapse to smallest only, if browser is very short, regardless of width.) thanks to Qaanol on the xkcd forums for finding all 17 panels
· Animationthanks to WolfieMario on the xkcd forums
· References:
Lump in snake... The Little Princethanks to various
Blue/orange rings... Portal 2

Rare. Unknown - Landscape mobilethanks to martndemus
· martndemus: "[this] is the one you would get if it didn't know anything about you"

No strong leads yet on why some people seem to default to the Snake comic and others to the Tortoise, Dinosaur, or Landscape. Location reports vary greatly for each one, so they are either not location-based or they are large clumps of areas. The Tortoise may be more common in Restoftheworld (non-US); the Snake seems more evenly distributed everywhere; the Landscape seems to appear in flat areas more; the Dino is least common, and seems to happen mostly/only in The States (US).
· Some people have reported getting the same one across multiple browsers, while it changes between browsers for others. Interestingly, it seems to stay consistent for one user across time, which together suggests that the server is neither randomizing nor using just one piece of data, but combining multiple pieces of data (e.g. location+browser) to determine the comic. (This may also explain individuals' variation from Unknown comics to location-specific comics, which is very common.)
· (Also note: where location data is shown, many locations have several data points, not just one. For example, with Tortoise: most states came up a few or several times, but other states did not come up at all. Not the expected distribution if it affects all states.)
· Can anyone challenge this theory?

Twitterthanks to geary
Alt text: "Umwelt is the idea that because their senses pick up on different things, different animals in the same ecosystem actually live in very different worlds."
(a shorter version of the full, perhaps because of the site's nature of brevity. Though, this is 153 characters... too long for a tweet.)
Notes: misspelled "aggression"

Card Counting: College and last panel text depends on location:HarvardleblanckMITforgot & vantilagevantilage: "course 15s refers to management majors"Smithkestrad@xkcdWellesleygoogledocs
(looks like he forgot to add hair to panels #2, #3, and #4)

It appeared possible to get all of the comics by editing short URLs early on April 1st. It appears that this was changed to a very long hash(?) value soon after to obscure all of the possibilities. Link

I got it on firefox no matter what I do. Tried private browsing and changing the resolution and what not and all I get is the one I posted. I live really close to the lake though(Vergennes/Ferrisburgh) so maybe that's why?

The one marked as "Florida" (re: earthquakes) must have some sort of redirect; in Wisconsin, I got the same one, except it says "Wisconsin" instead of "Michigan". I would guess that one should be identified as either Midwest or Upper Midwest.

EDIT: Got the one described above while using Opera, using Firefox from the same location gave me the Canadian aurora comic.

RE-EDIT: okay, more clarification: the one listed above currently marked "Florida" says "But this is MICHIGAN!" and ends with a blizzard. There's one further in this thread that say "But this is FLORIDA!" and ends with a hurricane. It looks like most of the panels are recycled, with the joke being "substitute local disaster that locals take for granted", which disaster depending on the locale.

RE-RE-EDIT: Okay, seeing the refresh of the parent, blizzard would be upper midwest, tornados would be midwest, hurricanes for Florida/SE US?

Edit: Go to xkcd and view source. 2nd to last line of the page is a javascript called "waldo.js"
The script pulls all the data from this source: http://umwelt.xkcd.com/story/ghenkEggov8
I assume the last 11 characters in that URL change based on location. I.E. the js file is made dynamically in php and the php stores different variables in a database and the file serves up the database row in json encoded format.

Someone commented somewhere that his coworker is friends with Randall, and he was totally holed-up working on this for a full week prior.

I'm guessing the logic behind the unknowns is actually pretty simple to state, like "add the digits of the IP and the alphanumeric value of the browser and modulo 4 to determine default comic", but very hard to determine without an enormous set of data (like thousands or millions) to analyze computationally.

Of course, that's not to say the whole thing was easy to orchestrate. :)

After reading like 2000+ comments, this is easily one of the most interesting, if it's true. All this time, we've been speculating that the change in comics is based off of location, browser, or some combination of settings like those. If your phone switches without even having to reload, then the javascript must be randomizing it to some extent.

If you can get it to give you two different comics again without switching any variables (especially without a reload), let me know! :)

I'm trying to remember what I saw. I think it went like this:
1) Phone in portrait mode, dinosaur comic showing
2) Flip phone to landscape mode, dinosaur comic shows for a moment
3) Dinosaur comic disappears, leaving just the title Umelt, with the image loading...
4) Snake comic appears

Interestingly, today on the phone I only get the snake image. I'm on a different wifi network from the two previous times I reported. Weird.

Each time, I'm getting to the comic by searching for "xkcd" from Android's main internet searchy feature, using the default browser, and clicking the first link.

Cool, well thanks for trying it again. My best guess still isn't all that great for how the server's switching between the four seeming "default" comics and/or location-specific comics, so this information helps.